Topic: Cb750 B750A carb thoughts ..

[Salman_z8]

Not exactly a problem, but what float height and jetting are you folks using for a bone stock bike.
Also, i keep reading the 3-4mm below the bowl seam, but in my experience at 26mm, with brass floats and all factory carb parts, it is always much lower than that. More like 10-12mm

Anyone running a bone stock 69 diecast, what jetting are you running, factory ?
120main, 40 pilot, ??

Now that i have two 69’s together, i am thinking about more precision tuning.
I am sure that atleast one of my bikes has the original cam, the other one i cant remember what i put in, may be k4-k6.

One has no number pipes, one has hm300

[Salman_z8]

Funny thing is i always used to measure float heights from the gasket surface, much later i started measuring from the notch.
Always had much easier starting and better midrange with the higher fuel level, never really measured how how it actually was, but bikes ran fine, started fine, idled fine etc

I personally have never clear tubed, as a means to set fuel level, but now that i have, my OCD has kicked in, and the pursuit of as good a performance as that engine is capable of.

[Salman_z8]

This is the level measured using the clear tube,
While the float being set at 26mm from the gasket  seating surface, carbs tilted, just touching the valve

It still isnt 3-4mm … are my float levels incorrect or is everyone getting the same ?

I certainly need to go a smaller main, 120 is rich up top

Cylinder 1 and 4, 1 being a bit higher, although both were set the same

[Salman_z8]

Can someone chime in and tell me, if they set there brass floats at 26mm, what is the correct level of fuel that shows up in the clear tube ?

I have now tried it a few different ways.
But i get pretty rich if i move the fuel up, at the level posted in the photo, i am rich, and barely ever need choke to start.
Almost no cold bloodedness, lots of low + midrange power, but it is rich.

[newday777]

Can someone chime in and tell me, if they set there brass floats at 26mm, what is the correct level of fuel that shows up in the clear tube ?

I have now tried it a few different ways.
But i get pretty rich if i move the fuel up, at the level posted in the photo, i am rich, and barely ever need choke to start.
Almost no cold bloodedness, lots of low + midrange power, but it is rich.

I haven't had a 69 to play with and can't answer your question on it.

You might also try asking this in the sohc/4 bikes section as you aren't supposed to ask technical questions in the project shop section(so the project shop section doesn't get cluttered) Read all the sticky posts(there's only 4)at the top of the project shop section, on what and how to post your project all in one thread, not individual posts apart from your main project thread on the project bike.

http://forums.sohc4.net/index.php/board,2.0.html

[HondaMan]

The 'B750' carbs should have a tiny sliver of aluminum standing adjacent to the outer notch, but inside the float gasket. This is the float reference height for setting the floats, not the outer notch (nor the float gasket's surface). Then, the float nearest the kickstand (in each carb) is set 1mm deeper at 25mm, and the opposite one is set at 26mm.

In the early carbs, check the hole sizes in the emulsifier tubes. They should be opened up a little bit to use our modern slow-burning fuels: the larger ones should be not less than 0.035" and the smaller ones 0.025". I usually open these up to at LEAST 0.0375", and more often to 0.039" because our fuel burns much slower today. And, use less octane, like Regular for in-town use and Mid-grade for mixed town/hiway riding. The only time I even use Premium nowadays is when riding on Western Interstates out here, which have 80-85 MPH speed limit signs. Today's Midgrade (and in some areas, even the Regular) gasolines burn at the speed of 1970s Premium grade, so it acts too rich for the most part.

In the engine with the post-1972 cam: use #105 or possibly #110 mainjet. Honda reduced the intake valve duration by 3 or more degrees in the 1974 engines, and also made the #2 & #3 intakes open even later than the #1 and #4 intakes, to smooth harmonic vibrations at 55 MPH in top gear, which was essentially almost lugging this engine(!). In the K5/6 cam (provided the K6 cam was not actually one from the F0, some which were) the exhaust valves close faster than in the earlier cams, which helped improve effective compression ratio above 3500 RPM or so. These 'tweaks' were done at the time to improve fuel MPG to meet the boasts of 50 MPG made in the [American] motorcycle magazines of the time. As a result, some of the late K5/6 cams don't make for stellar performance in an earlier engine unless the intake tract, spark advancer, and crankcase vents are also changed, and even then it will still have less top speed than in the original K1-K3 engines.

[Salman_z8]

Interesting.
I will check for that sliver.

I am trying to figure out, why with 40 pilot and 120 main, and middle clip, i am not rich already.

Lol

[HondaMan]

Interesting.
I will check for that sliver.

I am trying to figure out, why with 40 pilot and 120 main, and middle clip, i am not rich already.

Lol

In the B750 carbs the lifting efficiency of the needle jet is slightly less efficient than in the later 657A series of carbs. Honda started with the 120 mains and by the time the B750's throat was made more consistent in production the jetting had to be lowered. First it went to 115 briefly, then to 110 by the end of the K0 bikes. Then the 657A series started with 115 for a few months and went to 110 after that, with slightly different throats.

[Salman_z8]

Which sliver are you referring to mark ? Thanks, i think i am blind
I had another set of cleaned b750a on the bench 

[HondaMan]

Here's a picture from a [much modified, note the plastic floats] carb from an early K1 Old Factory bike. They had all these little "secrets" in those bikes back when.

I rebuilt a set of B750 carbs last winter for someone, and 3 of them had the sliver, while 1 did not. I wasn't sure if they are all virgin carbs from the same rack, but I found it odd that one was missing. Yours doesn't appear to have it, either?

Back when, I thought this little edge was there to hold the gasket away from the [brass] floats because they were real close together, especially if a brass float swelled (ballooned) out like they sometimes do (from very high altitude riding, like climbing 14-ers here in Colorado). I learned later on that wasn't the case.

This sliver was (when new) clearly flat-filed on its top edge, probably to the maker's jig height settings by a technician. That's the sort of things Honda used to do then to build their other bikes, so it was natural for it to show up in [at first] the 750. There are dozens of little things like this in the sandcast and K0 bikes, some which continued in the Old Factory K1 and K2 bikes, that vanished altogether in the New Factory versions. Sometimes you will find a vestige of these 'slivers' in the carbs, partially (or worse) dissolved by the years of MTBE additive in USA fuels in the 1980s. That acidic stuff was intended to shorten the life of the American Vehicle Fleet to [try to] speed up the "adoption" of vehicles with less emissions. It worked, too.